With respect to the coating equipment industry in general, there exists many different methods to achieve atomization of a coating material for the desired purpose of coating a workpiece. In one general class of spray guns, compressed air is used to atomize the coating material. The most common air atomizing spray gun uses air pressures on the order of 1.75 to 3.5 Kg/cm.sup.2 (25 to 50 psig) to achieve atomization, although pressures as high as 5.6 to 6.3 Kg/cm.sup.2 (80 to 90 psig) or more may be used. These spray guns use a high air velocity which can sometimes reach sub to supersonic speeds to atomize the coating. However, the high air velocities impart a high velocity to the atomized coating material which in turn causes "bounce-back" of atomized coating particles from the painted surface, hence a less than ideal transfer efficiency. Atomized coating material which does not transfer to the workpiece is dispersed in the atmosphere to cause potential pollution problems and also increases the cost of coating the workpiece.
HVLP is a method of atomization utilized in paint spray gun atomizers, both hand-held and fixed mounted guns. Atomization is achieved in a similar manner to the more commonly know higher pressure air atomizing gun. The major difference is that an HVLP spray gun may not operate at dynamic air pressure of greater than 0.7 Kg/cm.sup.2 (10 psig) measured upstream of the air nozzle. The 0.7 Kg/cm.sup.2 (10 psig) limit for HVLP equipment is set by statute in some jurisdictions, such as the state of California. Reasonably good atomization occurs with HVLP spray guns because the relative size of the air jets in the air nozzle are significantly enlarged over the more conventional spray gun to pass a greater volume of air at the 0.7 Kg/cm.sup.2 (10 psig) limit than could be obtained with conventional air atomizing spray gun at an equal pressure. The resultant higher volume of air tends to compensate for the lower upstream air pressure, and therefore approximately duplicates the available energy at the point of atomization. Owing to the lower available air pressure upstream of the air nozzle, as well as the larger air jets, the relative velocity of the air stream at the interactive point of atomization is considerably lower than with conventional air atomizing guns. This lower air velocity contributes to an overall "softer" spray cloud which, in turn, provides less bounce-back of the atomized coating material from the painted surface. Consequently, transfer efficiency increases and environmental concerns are reduced.
In one of the most common HVLP spray gun systems, the flow of high volume low pressure air is generated by an electrically driven vane compressor typically called a turbine compressor. The air is delivered to the gun via a relatively large diameter hose in the high volume, low pressure condition. In order to carry the required air flow volume at the low pressure, the hose typically has an internal diameter of at least 2 cm (1/4 inch) and may have an external diameter of on the order of 2.5 to 4 cm (1 to 11/4 inches). As a consequence of the work imparted on the air by the turbine compressor, the delivered air will usually have an elevated temperature. The air is directed through the gun and through a manually operated proportioning valve generally at the front end of the gun, directly behind the air/fluid nozzle. The purpose of the proportioning valve is to direct the air to separate fan air and atomizing air ets in the air nozzle in order to control the relative amounts of air going to the different jets. The valve is typically designed to maintain a constant flow rate, so that a specific desired increase in atomizing air flow will cause a corresponding decrease in fan air. The reason for this is that the turbine compressor is typically a positive displacement, non-pressure compensated, non-pressure relieving device. Careful proportioning of the fan and atomizing air are thus important in order to prevent undesirable increases in back pressure between the gun and the compressor which could create excessive heat generation in the compressor, resulting in shorter compressor life. Thus, the fan and atomizing air must flow constantly from the air nozzle regardless of whether the paint is triggered on or off. One major drawback to such a system is that the relatively large and cumbersome air hose results in operator fatigue and also limits maneuverability of the spray gun. In addition, the adjustment of the fan air and atomizing air must be achieved through a delicate balance of both a bleed valve at the compressor and the proportioning valve on the gun.
Many facilities have "plant air" available with a line pressure of about 5.6 Kg/cm.sup.2 (80 psig). Where plant air is not available, a conventional air compressor may be available. A variation of the above described HVLP paint spray system replaces the turbine compressor with plant air or another source of compressed air in the 2.8 to 6.3 Kg/cm.sup.2 (40 to 90 psig) range. A manually adjusted self relieving air pressure regulator can be used to drop a relatively low flow of the high pressure air to a higher volume flow of low pressure air and such HVLP air is delivered through a large diameter hose to the spray gun. A constant air bleed is no longer needed in order to protect the compressor, provided the source includes a regulator of the self-regulating design. Nor is an air proportioning valve needed. However, a cumbersome air hose of the type used with a turbine compressor operated HVLP system must be used between the regulator and the spray gun to carry the needed air volume at a low pressure. Further, adjustments must be made at both the gun and the regulator to achieve proper atomization and a desired fan pattern.
An improvement to the HVLP spray gun operated from a high pressure air source involves delivering the high pressure air through a small diameter air hose to the spray gun and passing the air through either a calibrated fixed orifice or through an adjustable orifice in the form of a needle valve. The resulting pressure drop across the orifice provides the required lower air pressure at an expanded volume. Where an adjustable orifice is provided, the orifice at the valve seat can be suitably sized so that at the maximum open position and at the maximum supply air pressure, a dynamic air pressure of 0.7 Kg/cm.sup.2 (10 psig) is never exceeded downstream of the needle valve. The advantage here is that a smaller compressed air hose, typically no greater than 1 cm (3/8 inch) inside diameter may be used, as compared to the significantly larger diameter air hose required to deliver a high volume flow of low pressure air. The disadvantages are that a needle valve can be clogged by foreign debris and that because this valve is not self-compensating, it is also negatively affected by changes in supply air pressure. Hence, a slight dip in supply air pressure will cause a corresponding undesirable dip in atomizing and fan air pressure at the air nozzle. Also, owing to the non-pressure compensating feature of the needle valve or fixed orifice, the orifice needs to be sized based on the various air nozzle versions for a given HVLP spray gun. For example, if the orifice size is determined based on a wide fan air nozzle, then using a narrow fan air nozzle (having less air flow) would cause a higher back pressure tending to exceed the 10 psig limit. Therefore, the orifice must be sized to accommodate the back pressure limitations of the lowest flow air nozzle, which then compromises the maximum attainable operating pressure with the higher flow, wider fan air nozzles. The result is that the wider fan air nozzles tend to provide a relative lower degree of atomization than the narrow fan air nozzles.
It is therefore desirable that an HVLP spray gun be capable of receiving the required air via a small diameter compressed air hose for increased maneuverability and to be able to convert the supplied high pressure air into a predetermined constant low pressure high volume air flow having a pressure which does not exceed 0.7 Kg/cm.sup.2 (10 psig). It is further desirable that such a spray gun not be affected by the kinds of external influences described above, such as debris in the air line, air supply pressure variations, and various air nozzle designs which permit different air flow rates for a given gun operating air pressure.